The present invention pertains to methods for removing hydrogen from various atmospheres within enclosed spaces which also contain significant concentrations of oxygen. In particular, the invention pertains to the use of new organic polymer formulation for scavenging hydrogen which is also capable of absorbing liquid water formed as the result of the catalysis of hydrogen and oxygen.
In many applications the presence of hydrogen and its isotopes, arising from various chemical and electrochemical corrosion reactions, can be detrimental. The accumulation of hydrogen can present a significant fire and/or explosion hazard, particularly in sealed components. Mechanical and electrical safety devices, such as pressure relief valves, can be expensive, unreliable, and particularly for small devices, not always practical.
It has long been known that certain gas scavenging materials, (also known as getters), can be used to counteract the accumulation of hydrogen and/or oxygen within closes spaced. In U.S. Pat. No. 4,512,721, Ayers et al., discuss the use of active metals such as zirconium or titanium, and alloys thereof, for maintaining low hydrogen partial pressures but have the disadvantage of requiring high temperatures for initial activation and/or ongoing operation because of the necessity to diffuse surface contaminants into the bulk metal thereby providing a fresh surface for continued hydrogen absorption.
Labaton, in U.S. Pat. No. 4,886,048, describes another means for removing hydrogen by reacting the hydrogen with oxygen to form water (recombination), in the presence of a noble metal catalyst such as palladium, and trapping the water on a water absorbing material such as a molecular sieve. However, hydrogen getters of this type are expensive, bulky, limited by the availability of oxygen, and capable of causing a detonation if improperly formulated.
It is well known also that unsaturated carbon-carbon bonds (i.e., double and/or triple bonds between carbon atoms) can be reduced by hydrogen, in the presence of an appropriate catalyst, to form an alkane (see, for example, Fieser, L. F. and Fieser, M., Textbook of Organic Chemistry, D. C. Heath & Co. 1950, pp. 66–69 and 86). See also Anderson et al. (U.S. Pat. Nos. 3,896,042 and 3,963,826); and Harrah et al., (U.S. Pat. No. 4,405,487).
In particular, commonly owned U.S. Pat. Nos. 5,624,598, 5,703,378, 5,837,158, and 6,063,307 to Shepodd, et al., (the teaching of which is herein incorporated by reference in their entirety) describe hydrogen getter systems that utilize double and/or triple bonded organic polymer compounds mixed with a hydrogenation catalyst, typically a noble metal selected from New IUPAC Groups 9 and 10 of the Periodic Table of Element.
The Shepodd, et al., references teach that when these systems are exposed to hydrogen, the unsaturated carbon-carbon bonds are irreversibly converted to their hydrogenated analog with the aid of the associated catalyst. Hydrogenation of a carbon-carbon double bond in an organic compound by means of a catalyst, as illustrated in FIG. 1, is typically an irreversible, exothermic, heterogeneous reaction. That is, hydrogenation (the addition of hydrogen to at least one unsaturated carbon-carbon bond) takes place at the boundary between the catalyst and the organic reactant. Consequently, in order to achieve the highest degree of effectiveness, the getter materials should preferably be prepared using techniques that ensure that the catalyst is in reactive association with the unsaturated carbon-carbon bonds of the organic reactant compound.
The heterogeneous nature of organic hydrogen getters causes a distribution of reaction sites that react at different rates. While in principle, the getter will not stop reacting until all the unsaturated bonds have been hydrogenated, in practice, the rate becomes vanishingly small as the getter approaches saturation. Furthermore, in order for hydrogenation of an unsaturated carbon-carbon bond to take place in an organic hydrogen getter it is necessary not only that the hydrogen gas but also the catalyst that mediates the reaction be proximate the unsaturated bond(s). Thus, in addition to ensuring that the catalyst is distributed as uniformly as possible throughout the organic hydrogen getter, it is necessary to provide for movement of the organic getter molecules such that as many as possible of the unsaturated bonds in the organic molecule are brought into effective contact with the catalyst. While this is relatively easy to accomplish for small organic molecules it is extremely difficult for long chain polymer molecules.
Shepodd, et al., however, discovered that by employing long chain organic polymer molecules having a low glass transition temperature (Tg), such as polybutadiene and its co-polymers, it was possible to formulate an organic hydrogen getter that possesses a high capacity for hydrogen absorption as well as a high hydrogenation efficiency. Having a low Tg endows the polymer molecules with fluid-like properties that permit movement of the polymer molecule itself, thereby bringing unsaturated bonds in the molecule into proximity with the catalyst to provide for hydrogenation of the unsaturated bond. Furthermore, lower weight polymer chains that can move more rapidly to a catalyst site demonstrate enhanced reactivities.
These prior art organic hydrogen getters and recombiners have shown themselves to be effective at removing hydrogen to trace levels in the presence of other gases within protected volumes. However, when oxygen is present, a competition is set up between the direct gettering of hydrogen (hydrogenation of carbon-carbon multiple bonds) and the catalytic recombination of hydrogen with oxygen to form water.
While these getter formulations are not generally affected by water formation, the surrounding elements of the apparatus protected by the getter may be adversely affected by liquid water. This is a particular problem in semi-open systems or voluminous containers where there is a large potential for liquid water formation because of the amount of oxygen present. Water vapor up to 100% humidity is not generally a problem; however, liquid water, even if it is formed as a vapor that then condenses as the result of temperature and/or pressure fluctuations, or prolonged recombination, can cause corrosion, electrical shorts or transport of hazardous materials within the protected volume. The presence of liquid water is, therefore, detrimental to the function and structure of many protected volumes.
Finally, other recombiners have been described that absorb the water onto desiccants proximate, or commingled, with the reactant species. These materials are distinct from the present invention because the desiccants utilized in these previous compositions will also absorb water vapor from the air and, therefore, can become saturated without ever contacting liquid water. The present invention only absorbs water as a free liquid. Finally, should these prior art compositions encounter sufficient quantities of liquid water to submerge the getter, the reaction rate of the getter is dramatically reduced, becoming limited by the diffusion rate of hydrogen through the water layer.
What is desired, therefore, is a polymer formulation comprising a long chain organic molecule having a plurality of unsaturated carbon-carbon bonds. It is also desirable that the polymer formulation exhibit a high efficiency for removing hydrogen and oxygen, is relatively inexpensive and readily available, and is capable of absorbing liquid water and still function as an effective getter for hydrogen and oxygen.